Although many studies have attempted to define appropriate nursing staff levels, allocation and patient dependency, minimal data is available on the effect of nursing staff shortage (NSS) on quality of care provided in intensive care. This study aimed to identify incidents associated with staff shortage as reported to the Australian Incident Monitoring Study-ICU (AIMS-ICU) project and to assess their estimated effect on patient outcome. A search of narrative keywords and contributing factors identified 89 nursing staff shortage incidents (NSS-INCIDENTS) and 373 incidents involving nursing staff shortage contributing factors (NSS-CF). NSS resulted from inappropriate rostering for current patient load (81%) and inability to respond to increased unit activity (19%). Most frequent associated incidents included problems with: drug administration/documentation (47), patient supervision (20), set-up of ventilators/equipment (16), and accidental extubation (14). Undesirable patient outcomes included: major physiological change (22%), patient/relative dissatisfaction (12%), and physical injury (3%). This study suggests that inadequate staffing results in incidents and compromised patient safety.